Personally, I only believe the death penalty is warranted in cases where someone is likely to either commit further crimes from within prison, or escape prison to commit further crimes. But then, I'm a pragmatist at heart, and punishment for the sake of punishment or vengeance really has no place in that. It's only useful if it accomplishes some worthwhile goal in the wider world.
The issue with any criminal who is violent for psychopathic reasons, as in like anyone who does this, looks like this, etc of them killing/harming for no personal vendetta against the person but other reasons that cause them to dislike the person might have that reason come up while in prison and hurt/kill other prisoners.
Short version:If they are in prison for life, what reason do they have to not do bad things in prison? They are already in prison with no hopes of getting out so what else can you do to them but death, which might not be that important to them since they have to rot away in prison, anyways.
